The apparatus and methods described herein relate generally to tank mixing systems and, in particular, to tank mixing systems for sludge storage tanks and digester tanks requiring surface mixing.
Storage tanks are often used for municipal and industrial sludge and other applications, such as storing sludge from municipal and industrial waste treatment facilities. The sludge generally comprises both solid and liquid components. The storage tanks may be used for storing the sludge when received from a waste treatment facility prior to processing and after processing. In addition, storage tanks may be used for treatment processes, such as aerobic and anaerobic digestion. The storage tanks are typically large, ranging from about 10 feet in diameter up to and beyond 150 feet in diameter. The depths of such tanks likewise have a broad range, varying between about 10 feet to about 40 feet and above.
Due to the mixture of liquid and solid components forming the sludge, and the large volumes of sludge frequently present in the tanks, settling of the solid components relative to the liquid components often occurs. The solid components of the sludge tend to settle in a layer toward the bottom of the tank over time, while the liquid contents remain above the accumulated solid layer on the bottom floor of the tank. In order to facilitate removal and/or further processing of the sludge in the tank, including both liquid and solid components, it is desirable to break up the solid layer on the bottom floor of the tank and resuspend the solid components into the liquid components. Such resuspension involves mixing of the tank contents to move the solid components from the floor in order to create a generally homogenous liquid and solid slurry within the tank. A variety of mixing systems aimed at suspending the solid components back into the liquid components of the sludge have been developed. In some instances, flow patterns are developed within the tanks in order to mix the solid and liquid components of the tank contents together in an efficient and effective manner. One such system is disclosed in U.S. Pat. No. 5,458,414.
During the mixing process, gas entrapped in the solid components often causes large chunks of solid debris to rise toward the surface of the tank and even float on the surface of the tank contents, particularly as the solid layer on the tank floor is broken up. Solid debris floating on the surface of the tank in large chunks is undesirable because mixing processes can occur more efficiently beneath the surface of the liquid tank contents. Solid debris on the surface can be difficult to break up and resuspend into the liquid. When flow patterns are developed in the tank contents, it is desirable to have the solid debris submerged for entrapment in the flow pattern to break up the solid debris. Floating solid chunks can reduce digestive capacity and performance, may result in plugged pipes and pumps, and generally inhibit mixing of the tank contents.
Scum layers may also form on the surface of tank contents during the mixing process. Scum layers might appear on the liquid surface of anaerobic digesters and contain grease, vegetables and mineral oils, and other floating materials such as hair, rubber goods, animal fats, bits of cellulose material, pre-fatty acids, and calcium and magnesium soaps. Scum accumulations can have a specific gravity less than the specific gravity of the sludge, causing the scum to rise toward the surface of the tank contents and even float on the surface.
When the scum accumulations are floating on the surface of the tank contents, it is very difficult to break up or entrap them in the flow pattern beneath the surface of the tank. The scum layers can vary in size from a few inches to several feet in depth. The depth of the scum layer and degree of solidification depends on a variety of factors, such as the volumes of grease and oil in the sludge in the tank, whether sedimentation in the tank is treated separately, the temperature of digester contents, the degree and type of tank mixing, the frequency of cleaning, and whether a tank has a fixed or floating cover. The scum, similar to solid debris floating on the surface, is undesirable because it is difficult for typically submerged tank mixing systems and flow patterns to adequately mix the scum layers and suspend the solid components thereof into the liquid for facilitating removal from the tank or further processing.
In addition to scum, foam can also develop on the liquid surface in anaerobic digesters. Foam can be caused by high grease content, inadequate mixing, a high percentage of activated sludge in food, sludge thickening by dissolved air floatation, several temperature fluctuations, high CO2 content, high alkalinity, low total solids, excessive mixing rates, and high organic content in the food sludge. Foaming is similar to scum except foam typically has entrapped gases that causes the foam, and the contents thereof, to rise to the surface of the tank. Foam, similar to solid chunks and scum, presents a problem for tank storage systems because it is difficult to break up the foam layer and resuspend the solid contents thereof into the liquid solution for facilitating removal from the tank or further processing. A variety of approaches have been developed for attempting to address foam and scum control. For example, when foam and scum is developed due to excessive grease, grease can be removed from the process train using primary clarifiers. However, the use of primary clarifiers in order to remove the grease complicates the tank storage system and increases the cost.
Another solution developed in an attempt to address foam and scum accumulation problems is to continuously mix the contents of the tank to reduce settling of the solid components. However, mixing continuously can be inefficient and can result in even more scum and foam production when excessive mixing rates are used. Rapid mixing can lead to an increase in entrapment of gasses associated with foaming in solid components, resulting in an increase in foam and scum production.
Other complicated methods of attempting to reduce scum and foam involve minimizing temperature fluctuations. However, temperature variations of just two to three degrees Fahrenheit can cause foam problems. Therefore, controlling foaming by reducing temperature variations can be impractical. Scrubbing digester gases to remove CO2 has been done in the past but requires expensive and complicated scrubbing mechanisms. The use of actinomycetes have also be used, but requires time intensive and trial and error experimentation and may not be reproducible due to the large variations in the characteristics in the tank contents frequently present.
In some instances, the use of nozzles positioned above the surface of the tank can be used to break up scum and foam layers present on the surface thereof. Such nozzles require manual operation, such as an operator positioned above the tank on a platform and aiming and directing a fluid stream from the nozzle at the foam and scum deposits on the surface of the tank in a random manner. Typically, the nozzles are rotatably and pivotably mounted allowing an operator to aim the fluid stream as needed at the solid components present on the surface of the tank to break them up and urge them back under water where they can be effectively mixed by the tank mixing system. The nozzles can be problematic due to the requirement of an operator to selectively aim the fluid stream at solid deposits, scum and foam. Not only are the nozzles inefficient due to the increased time and operator effort that must be expended in order to break up the sludge deposits, which can take several hours, but the pumping energy required to pump fluid and discharge fluid through the nozzle can add to the increased cost of operating the tank storage system by substantially disrupting the fluid flow patterns within the tank. Moreover, such nozzles are impractical for use with covered storage tanks, where operator access is often impossible.
There is provided a new improved method and apparatus for mixing the liquid and solid components of the contents of a tank using a tank mixing system. This is achieved by using a flow generating device positioned to discharge a stream of fluid toward the surface of the tank to break up solid components present at or near the surface in a generally predeterminable region, which provides the improved result of breaking apart or otherwise mixing the solid components present at or near the tank surface for the purpose of facilitating mixing of the tank contents.
The tank may be generally circular in shape having an outer surrounding wall with a radius extending from the center of the tank to the outer surrounding wall. The tank is at least partially filled with contents having both solid and liquid components to a liquid level having a surface. A sump may be provided for withdrawing at least some of the contents from the tank. A pump may be provided having its input connected to the sump for withdrawing at least some of the contents of the tank through the sump. At least one submerged flow generating device, such as a nozzle or a propeller, is positioned within the tank and operatively connected to a discharge of the pump for pumping some of the contents through the submerged flow generating device to rotate the tank contents in a generally circumferential direction. An upper flow generating device, such as nozzle, may be positioned at an elevation above the liquid level of the tank contents and aimed to selectively discharge at least some of the contents into the tank at a downward angle relative to the surface of the liquid contents and tangent to a generally circular band on the surface between the tank outer surrounding wall and the center of the tank.
According to one aspect, the location of the generally circular band is between about 2% and about 50% of the tank radius inward from, the tank outer surrounding wall. The characteristics of the pump discharging fluid from the flow generating device and the diameter of the tank in part results in an energy gradient within the tank. The location of the generally circular band may be in part dependent upon the energy gradient within the tank. For example, when the energy gradient is below 80 horsepower per million gallons the location of the generally circular band may be between about 2% and 20% of the tank radius inward from the outer surrounding wall of the tank. When the energy gradient within the tank is above 80 horsepower per million gallons the location of the generally circular band may be between about 20% and about 50% of the tank radius inward from the outer surrounding wall of the tank.
The upper flow generating device may be elevated above the surface of the tank contents, and may be elevated about 10 feet above the surface of the tank contents. The upper flow generating device may be attached relative to the tank outer surrounding wall or, if the tank has a roof, to the roof of the tank. A platform may be provided for the upper flow generating device to be mounted on. The upper flow generating device may also be mounted on a preexisting platform, particularly when retrofitting existing tanks already having elevated platforms with the mixing system in accordance herewith.
The upper flow generating device is operatively connected to a pump that withdraws at least some of the contents from the tank for discharge through the upper flow generating device. The pump may be the same pump for the submerged nozzles. The discharge rate of fluid through the upper flow generating device may be dependent in part upon the energy gradient within the tank. The upper flow generating device may have a discharge rate of between about 100 gallons per minute and about 500 gallons per minute. The tank contents may have a volume and the discharge rate of the upper flow generating device may be selected to be between about {fraction (1/10)} of a percent and {fraction (1/30)} of a percent of the contents volume.
Another system is provided from mixing the liquid and solid contents of a tank. The system includes an outer surrounding wall of the tank for at least partially containing the solid and liquid components therein. At least one flow generating flow generating device, such as a nozzle, propellor, or other suitable apparatus, is positioned to discharge fluid into the tank for creating a fluid flow within the tank. The fluid flow has a flow moving the contents of the tank in a direction of rotation in addition to having a generally inward component and a generally outward component proximate the surface of the tank contents. The generally inward and generally outward components of the fluid flow meet in a region of the tank. A surface flow generating device, such as a nozzle or other suitable apparatus, is oriented above the tank contents to downwardly direct a fluid stream onto the surface of the tank contents at the region of the tank where the generally inward and outward components of the fluid flow meet.
The tank may be generally circular and thus the outer surrounding wall may also be generally circular and located at a radial position from the center of the tank. The surface of the tank contents extend to a height above the floor of the tank.
The flow generating device may be submerged beneath the surface of the tank contents and the surface flow generating device may be positioned a distance spaced above the surface of the tank contents. A pump having a pumping rate may be operatively connected between the tank and the flow generating device for drawing at least some of the contents from the tank and discharging them through the flow generating device to create the fluid flow.
The region of the tank where the generally inward and generally outward components of the fluid flow meet may be a generally circular band positioned between the outer wall and a center of the tank at a predeterminable location. The location of the generally circular band may be determined based in part upon the pump rate, the viscosity of the tank contents, the tank radius, and the height of the contents within the tank. When a portion of the solid contents are present on a surface of the tank contents within the generally circular band, the surface flow generating device is positioned to discharge the fluid stream to contact the portion of the solid contents. The contact between the fluid stream and the portion of the solid contents may break up the portion of the solid contents for submergence beneath the surface of the tank contents and for entrapment into the fluid flow within the tank. The fluid stream of the surface flow generating device may be positioned at an angle relative to a radial line extending from the tank center to the tank outer wall. In addition, the surface flow generating device fluid stream may be directed in the direction of rotation of the tank contents to minimize disruptions in the fluid flow.
The fluid flow may include the flows described in U.S. Pat. No. 5,458,414, the disclosure of which is hereby incorporated by reference in its entirety. The fluid flow may include a flow toward the outer portion of the tank in the lower portion of the tank, upward in the outer portion of the tank, inward in the upper portion of the tank, and downward in the inner portion of the tank. These flows may be repeated as the contents flow in the rotational flow pattern.
A method is also provided for mixing the liquid and solid contents of a tank having a outer surrounding wall. The method includes discharging a stream of fluid into the tank through one or more discharge nozzles. The method also includes creating a fluid flow within the tank using the fluid discharged through the one or more submerged nozzles at a fluid discharge rate. The fluid flow has a generally inward component and generally outward component present near a surface of the tank contents. The generally inward and generally outward components of the fluid flow meet in a region of the tank. The method further includes directing a fluid flow from a surface nozzle onto the surface at the region of the tank where the generally inward and generally outward components of the fluid flow meet. In a further aspect of the method, the method includes determining the location of the region of the tank where the generally inward and outward components of the fluid flow meet based upon the tank size, the contents, characteristics and the fluid discharge rate. The step of creating a fluid flow may also include inducing a rotational flow of the tank contents with the one or more discharge nozzles. The step of directing a fluid flow may also include aiming the surface nozzle in the direction of rotation of the tank contents.